ABSTRACT ? Project 2 Genome instability refers to the tendency of the genome to undergo a range of irreversible DNA mutations, from base substitutions to chromosomal alterations. In contrast to DNA damage, DNA mutations cannot be repaired and have been implicated as a cause of aging since the 1950s. However, it has proved very difficult to test whether genome instability underlies, at least in part, the gradual loss of function and increased degenerative disease risk associated with aging. This is because somatic mutations are difficult to detect in normal, non-clonal tissues. New, single-cell whole genome sequencing technology, developed by Project 2 as part of this PPG, makes it possible to study different types of somatic mutations and their distribution across the genome directly in human or mouse tissues. In Aim 1, Project 2 will first test the effects on genome instability of rare genome maintenance genotypes associated by Project 4 with accelerated or delayed aging in humans. Then, with Project 1, we will test the possible functional impact of genome instability on human aging (Aim 2). Finally, with Project 3 we will test the possible role of genome instability in cellular senescence (Aim 3). The results of Project 2 should, for the first time, provide insight into genome instability in human primary cells as a function of age and its possible causal contribution to aging.